Examples

// ===== 1. SwiftUI counter =====
import SwiftUI

struct CounterView: View {
    @State private var count = 0
    var body: some View {
        VStack(spacing: 16) {
            Text("Clicks: \(count)").font(.title)
            Button("+1") { count += 1 }.buttonStyle(.borderedProminent)
        }
        .padding()
    }
}

// ===== 2. List with NavigationStack =====
struct UsersView: View {
    let users = ["Alex", "Sam", "Lee"]
    var body: some View {
        NavigationStack {
            List(users, id: \.self) { name in
                NavigationLink(name, destination: Text("Profile of \(name)"))
            }
            .navigationTitle("Users")
        }
    }
}

// ===== 3. Async networking =====
struct GitHubRepo: Decodable {
    let name: String
    let stargazers_count: Int
}

func fetchRepo() async throws -> GitHubRepo {
    let url = URL(string: "https://api.github.com/repos/apple/swift")!
    let (data, _) = try await URLSession.shared.data(from: url)
    return try JSONDecoder().decode(GitHubRepo.self, from: data)
}

// SwiftUI .task:
struct RepoView: View {
    @State private var repo: GitHubRepo?
    var body: some View {
        Group {
            if let repo { Text("\(repo.name): \(repo.stargazers_count) stars") }
            else { ProgressView() }
        }
        .task { repo = try? await fetchRepo() }
    }
}

// ===== 4. UserDefaults persistence =====
@AppStorage("theme") private var theme = "light"

struct SettingsView: View {
    @AppStorage("theme") private var theme = "light"
    var body: some View {
        Picker("Theme", selection: $theme) {
            Text("Light").tag("light")
            Text("Dark").tag("dark")
        }
    }
}

// ===== 5. Error handling =====
enum APIError: Error { case notFound, network(Error) }

func loadUser(id: Int) async throws -> User {
    do {
        let (data, _) = try await URLSession.shared.data(from: URL(string: "/api/users/\(id)")!)
        return try JSONDecoder().decode(User.self, from: data)
    } catch let e as URLError {
        throw APIError.network(e)
    } catch {
        throw APIError.notFound
    }
}

// ===== 6. Form =====
struct SignInForm: View {
    @State private var email = ""
    @State private var password = ""
    var canSubmit: Bool { email.contains("@") && password.count >= 8 }
    var body: some View {
        Form {
            TextField("Email", text: $email).keyboardType(.emailAddress)
            SecureField("Password", text: $password)
            Button("Sign in") { /* submit */ }
                .disabled(!canSubmit)
        }
    }
}

// ===== 7. Concurrent tasks =====
func fetchAll() async throws -> [GitHubRepo] {
    async let a = fetchRepo()
    async let b = fetchRepo()
    return try await [a, b]
}

// ===== 8. Codable + JSON =====
struct Order: Codable {
    let id: UUID
    let total: Decimal
    let customer: String
    let createdAt: Date

    enum CodingKeys: String, CodingKey {
        case id, total, customer
        case createdAt = "created_at"
    }
}

let json = #"{"id":"...","total":49.95,"customer":"Alex","created_at":"2024-04-10T03:14:00Z"}"#
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601
let order = try decoder.decode(Order.self, from: Data(json.utf8))

// ===== Patterns =====
// - @State / @Observable for view state
// - .task for async work tied to view lifecycle
// - @AppStorage for small key-value persistence
// - Codable + JSONDecoder for networking
// - async/await + structured concurrency

// ===== Pitfalls =====
// - Force-unwrap (!) -> crashes
// - Long async chains without cancellation
// - Decodable mismatches (snake_case vs camelCase) without CodingKeys
// - State mutation outside main actor in SwiftUI

// Pure Swift CLI, then SwiftUI sample app, then a network + Codable refactor.